1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus incorporating the fixing device, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such image forming apparatuses may include an on-demand fixing device, which is heated up to a proper fixing temperature within a shorter time after the fixing device is turned on. The on-demand fixing device may include a fixing film, a pressing roller, a heating plate, an infrared heater, and a reflection plate. The heating plate provided inside a loop formed by the fixing film is pressed against the pressing roller via the fixing film to form a nip between the fixing film and the pressing roller. The heating plate is heated by the infrared heater provided inside the loop formed by the fixing film, and heats the fixing film at the nip. As a recording medium bearing a toner image passes through the nip, the fixing film and the pressing roller apply heat and pressure to the recording medium to fix the toner image on the recording medium.
As a structure to cause the infrared heater to heat the fixing film more effectively, the reflection plate covers a part of an outer circumferential surface of the infrared heater in a circumferential direction of the infrared heater along an axial direction of the infrared heater that is substantially perpendicular to the circumferential direction of the infrared heater. Thus, the reflection plate reflects light emitted by the infrared heater toward the heating plate. In other words, the fixing film is heated via the heating plate both by light emitted by the infrared heater and irradiating the heating plate directly and by light reflected by the reflection plate toward the heating plate.
The image forming apparatus forms a toner image on various sizes of recording media. Accordingly, in the fixing device, the fixing film and the heating plate have a width, in an axial direction of the fixing film perpendicular to a recording medium conveyance direction, which corresponds to a width of a maximum-size of recording medium that the image forming apparatus can accommodate. When the fixing device is turned on, the fixing film is heated along the whole width thereof.
With this structure of the fixing device, however, after small-size recording media pass through the fixing device continuously, heat is drawn from a center portion of the fixing film in the axial direction of the fixing film over which the small-size recording media pass and is thus cooled by successive passages of the small-size recording media. By contrast, both end portions of the fixing film in the axial direction of the fixing film, over which the small-size recording media do not extend and therefore do not pass, are heated up to an excessively high temperature because there is nothing to draw heat therefrom. Consequently, when a large-size recording medium passes through the fixing device, a toner image on the large-size recording medium is heated excessively at both end portions of the fixing film in the axial direction of the fixing film, generating hot offset.
To address this problem, the fixing device may include a plurality of infrared heaters corresponding to various sizes of recording media. For example, the fixing device may include a first infrared heater for heating the center portion of the fixing film in the axial direction of the fixing film and a second infrared heater for heating both end portions of the fixing film in the axial direction of the fixing film. However, with such an arrangement, disposition of the reflection plates corresponding to the plurality of infrared heaters may be complicated, resulting in degraded heating efficiency for heating the fixing film, an enlarged fixing device, or increased manufacturing costs of the fixing device.